In a conventional refrigerating machine having an expansion valve, a plurality of systems including a hot gas bypass system have been proposed to defrost an evaporator. Such systems are widely used in refrigerating machines or air conditioners for home use or official use (see, for example, non-patent document 1).
FIG. 12 is a block diagram of a refrigerating machine of the conventional hot gas bypass system as disclosed in the non-patent document 1.
In this refrigerating machine, a compressor 1, a radiator 2, a throttling device 14, and an evaporator 4 are connected in the form of a loop, and a bypass circuit 6 having an on-off valve 7 is interposed between an outlet of the compressor 1 and an inlet of the evaporator 4. During normal operation, a refrigerant is sucked into and compressed by the compressor 1, and the refrigerant discharged from the compressor 1 is cooled by the radiator 2 and discharged therefrom. The refrigerant is then reduced in pressure by the throttling device 14 and expands consequently. Upon evaporation in the evaporator 4, the refrigerant is again sucked into the compressor 1. During defrosting operation, when the on-off valve 7 is opened, the refrigerant discharged from the compressor 1 is led into the evaporator 4 through the bypass circuit 6 that bypasses the radiator 2 and the throttling device 14. Accordingly, the high-temperature refrigerant flows thorough the evaporator 4 and, hence, increases the temperature of the evaporator 4, making it possible to defrost the evaporator 4.
In recent years, however, a power recovery cycle has been proposed having an expander in place of the expansion valve in order to further enhance the efficiency of the refrigerating cycle. In this power recovery cycle, the expander acts to recover, when the refrigerant expands, pressure energy in the form of electric power or mechanical power, thereby reducing the input of the compressor by the amount of being recovered (see, for example, patent document 1).
FIG. 13 is a block diagram of the conventional refrigerating machine as disclosed in the patent document 1.
In the refrigerating machine as shown in FIG. 13, a compressor 1 is driven by a drive means (not shown) such as, for example, an automobile engine to suck and compress a refrigerant. The refrigerant discharged from the compressor 1 is cooled by a radiator 2, which in turn discharges the refrigerant towards an expander 3. The expander 3 then converts expansion energy of the refrigerant into mechanical energy (rotational energy) so that the mechanical energy (rotational energy) recovered may be supplied to a generator 5 for generation of electric power. The refrigerant that has been reduced in pressure and has expanded in the expander 3 evaporates in an evaporator 4 before it is again sucked into the compressor 1.
FIG. 14 is a Mollier diagram of the refrigerating machine of FIG. 13.
In the refrigerating machine, because the expander 3 reduces the pressure of the refrigerant while converting expansion energy into mechanical energy, the refrigerant discharged from the radiator 2 reduces enthalpy while undergoing a phase change along an isentropic curve (c→d), as shown in FIG. 14. Accordingly, as compared with a case wherein during pressure reduction the refrigerant merely undergoes adiabatic expansion without doing expansion work (an isenthalpic change), the phase change along the isentropic curve can increase enthalpy at the evaporator 4 by an amount corresponding to expansion work Δiexp, making it possible to increase the refrigerating capacity. Also, because mechanical energy (rotational energy) can be supplied to the generator 5 by the expansion work Δiexp, the generator 5 can generate electric power corresponding to Δiexp, which is in turn supplied to the compressor 1. As such, electric power required for driving the compressor 1 can be reduced and, hence, the coefficient of performance (COP) of the refrigerating cycle can be enhanced.
Further, in the power recovery refrigerating machine referred to above, a proposal has been made wherein a bypass expansion valve is provided in a circuit employing an expander and a generator separated therefrom (see, for example, non-patent document 2).
Another refrigerating cycle has been proposed wherein an expander and a compressor are connected to each other via a shaft so that energy recovered by the expander may be utilized to drive the compressor. In this refrigerating cycle, in order to avoid a limitation of the constant density ratio, a bypass circuit for bypassing the expander and a control valve for controlling the flow passage area of the bypass circuit are provided wherein the control valve for the bypass circuit is fully opened at the start of the cycle (see, for example, patent document 2).
Non-patent document 1: Closed Refrigerating Machine, 1981, ISBN4-88967-034-3 (pages 278-280)
Non-patent document 2: “Leading Study and Development of Basic Technology for Effective Utilization of Energy, Development of Two-Phase Flow Expander/Compressor for CO2 Air Conditioner” a 2002 Report by New Energy and Industrial Technology Development Organization
Patent document 1: Japanese Patent Publication No. 2000-329416
Patent document 2: Japanese Patent Publication No. 2001-116371